An otter pup at the Pueblo Zoo died due to severe brain inflammation.
The pup and two others were born March 13.
The deceased pup was found in the exhibit last week, but zoo officials said in a news release that they waited to release that information until they knew more about why the pup died.
The female pup, Saga, died of encephalitis, which the zoo says is an acute inflammation of the brain.
The other two pups appear to be healthy, according to the zoo.
"The zookeeper staff continues to monitor the otter family; however, we are cautiously optimistic that this an isolated event," the zoo said in a statement. "The other two pups seem happy and healthy."
Veterinary experts from CSU in Fort Collins are working with the zoo to find out the root cause of Saga's fatal illness.